In prior art dampening units are used in offset printing presses. It is the function of the dampening unit to apply fountain solution evenly on the printing plate of the offset printing presses. The fountain solution is applied to the first roll, for example, by means of a water chamber, nozzles or similarly. Via various other rolls, the roll usually applies a water film to the printing plate as evenly as possible.
The spray dampening units which use fountain solution nozzles to spray the fountain solution on the roll involve the problem that the fountain solution nozzles can be clogged with particles and be closed. Because of low pressure in the area of the nozzles resulting from the flow conditions in the area of the fountain solution nozzles, such particles, for example lint and ink particles from surrounding air, are being absorbed.
Clogged fountain solution nozzles must be cleaned resulting in expensive disruptions of the printing operations.
In order to deal with this problem, prior art has come up with a solution according to which, by means of compressed air, an air cushion is produced around the fountain solution nozzles so that the pressure conditions in the area of the nozzles prevent absorption and clogging of particles. For this purpose, nozzles can be provided around the fountain solution nozzle which blow compressed air in the discharge direction of the fountain solution nozzle, producing the air cushion around the fountain solution nozzle by means of a flow layer of compressed air which flows away from the fountain solution nozzle and prevents contaminated air from entering the fountain solution nozzle. For example, this technology is the subject matter of EP 1 155 824 A2. This thought was further developed in prior art by providing and designing respective covers. At the same time, it seems that providing a cover around the fountain solution nozzle results in further optimizing the flow conditions and thus protecting the fountain solution nozzles against contamination. Possibly, the covers also decrease the amount of compressed air used and make its use more effective. But the applications do not mention this. Respective covers are disclosed in WO 03/097358 A1, WO 2005/000583 A1 and U.S. Pat. No. 6,928,924.
This technology has the disadvantage that an additional air supply is required in the area of the fountain solution nozzles. The generation of an air cushion can also have a negative effect on the fountain solution distribution. Furthermore, compressed air is a relatively expensive medium.